Field of the Invention
Embodiments of the invention relate to an organic light emitting display and more particularly to a structure of power supply lines of an organic light emitting display.
Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (hereinafter, abbreviated to “OLEDs”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, etc.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer and electrons passing through the electron transport layer move to the light emitting layer and form excitons. As a result, the light emitting layer generates visible light.
The organic light emitting display arranges pixels, each including the OLED in a matrix form, and represents a gray scale by controlling an amount of current flowing in the OLEDs. In the organic light emitting display, an amount of voltage change differs depending on an amount of current flowing in power supply lines of a display panel. The voltage change includes voltage drop and voltage rising. The power supply voltage is lower than an original input value by voltage drop, and is higher than the original input value by voltage rising.
The power supply lines include high potential cell driving voltage supply lines (hereinafter referred to as “ELVDD supply lines”) for supplying a high potential cell driving voltage ELVDD to a driving thin film transistor (TFT) of each pixel. If desired, the power supply lines may further include auxiliary power supply lines, such as initialization voltage supply lines (hereinafter referred to as “Vint supply lines”) for supplying an initialization voltage Vint to each pixel and reference voltage supply lines (hereinafter referred to as “Vref supply lines”) for supplying a reference voltage Vref to each pixel.
As shown in FIG. 1, the ELVDD supply lines may be disposed on the display panel along a Y-axis direction in which data lines of the display panel extend. Two pixels, which are positioned adjacent to each other in an X-axis direction, may share one ELVDD supply line with each other, so as to improve an aperture ratio. An amount of voltage change varies depending on a pattern of an image displayed on the display panel. For example, an amount of voltage change in a bright image pattern is greater than an amount of voltage change in a dark image pattern. In particular, as shown in FIG. 2, when a motion picture is implemented and a bright image pattern (A) surrounded by a dark image pattern (B) moves to the right (or the left) at a rapid speed, characteristics of the voltage change vary depending on an image pattern variation of the display panel. Therefore, a moving vertical crosstalk is generated in the display panel.
FIG. 3 shows a boundary of the bright image pattern (A) in a measuring pattern of the vertical crosstalk in detail. A voltage distribution on the power supply lines at the boundary of the bright image pattern (A) shown in FIG. 3 is substantially the same as that shown in FIG. 4. As can be seen from FIGS. 3 and 4, a degree of the voltage drop in the bright image pattern (A), i.e., patterns {circle around (c)} and {circle around (d)}, is greater than a degree of the voltage drop in the dark image pattern (B), i.e., patterns {circle around (a)} and {circle around (b)}. Further, even if both the patterns {circle around (c)} and {circle around (d)} belong to the same bright image pattern (A), degrees of the voltage drop in the patterns {circle around (c)} and {circle around (d)} may be different from each other depending on light emitting colors of pixels implementing each of the patterns {circle around (c)} and {circle around (d)}. A voltage difference between the power supply lines resulting from the voltage drop leads to undesirable luminance difference between the pixels, and thus the vertical crosstalk shown in FIG. 2 appears on the display panel.
As shown in FIG. 5, the ELVDD supply lines may be disposed in a mesh structure, so as to minimize an amount of the voltage change. In the mesh structure, the ELVDD supply lines are disposed on the display panel in the X-axis direction as well as the Y-axis direction. When the ELVDD supply lines are formed in the mesh structure, the display panel may be entirely burnt when a short is generated between the ELVDD supply lines and other lines at any position inside the display panel. Thus, the reliability of the organic light emitting display may be compromised. Further, when the mesh structure is used, a thermal transfer path in the X-axis direction greatly shortens. Therefore, as shown in FIG. 9, an ignition at any one crossing of the display panel may be easily transferred to another crossing adjacent to the one crossing.
The Vint supply lines and the Vref supply lines may be disposed as shown in FIG. 6, or may be disposed in the mesh structure as shown in FIG. 7 so as to improve an aperture ratio. When the Vint supply lines and the Vref supply lines are formed as shown in FIG. 6, the above-described problems resulting from the voltage change are generated. When the Vint supply lines and the Vref supply lines are formed in the mesh structure as shown in FIG. 7, a horizontal crosstalk is generated by capacitive coupling at crossings between the data lines and the auxiliary power supply lines.
As shown in FIGS. 8 and 9, a level of a data voltage Vdatal sharply changes at a boundary between a background pattern and a box pattern, each of which has a different gray level. In this instance, a capacitive coupling is generated between the data lines and the auxiliary power supply lines and thus swings the reference voltage Vref. Because the auxiliary power supply lines having the mesh structure are connected to one another in the X-axis direction as well as the Y-axis direction, the swinging reference voltage Vref is spread in the X-axis direction. A ripple component of the reference voltage Vref affects operations of all of the pixels positioned around a boundary of the box pattern and thus leads to the horizontal crosstalk.
Further, when a short is generated between the auxiliary power supply lines and other lines at any position of the display panel, a high short current resulting from a voltage difference between the shorted lines and a low short resistance locally flows in the display panel. Hence, heat is generated in the short point of the display panel. As shown in FIG. 10, when the auxiliary power supply lines are disposed in a mesh structure, such a heat may be transferred to left and right and top and bottom. Hence, a temperature around the shorted position of the display panel sharply increases, and the display panel may be entirely burnt.